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PostPosted: Sat Oct 26, 2013 11:17 pm 
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Location: San Antonio, TX
Gary B wrote:
Very good discussion (with no 100% clear conclusion though many valid observations)

I would only add that many molded case circuit breakers seem to clear instantaneous based on frame size and not breaker rating, so a small breaker rating might be misleading.




I agree with Gary B regarding that this is a good discussion without a clear conclusion. I called this the "Belly Button Effect". Everybody has a belly button and thinks is the best. The same with these opinions. Unless there is a clear guide from NFPA 70E or IEEE 1584 regarding this issue, it will remain just an opinion.

I believe that I have "proven" my opinion with research and experience. Hundreds of AFHA and many hours doing research with different scenarios. I would change in a heart beat if I would see another opinion based on research and examples. I have been wrong before. I could be wrong now.

Challenge:

Can anybody document several situations in which small 480V CB (less than 100A) with distances of less than 250 feet results in a AFIE substantially larger than 1.2 cal/cm^2?


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PostPosted: Sun Oct 27, 2013 12:05 am 
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haze10 wrote:
My point is that an 'assumption' at whatever size, should not be made. The primary basis for IE is going to be arcing current and clearing time. Without performing the fault analysis and examining the breaker's trip curve, there is no assurance that the breaker will be operating in its Instantaneous trip range. Fault currents can fall off quickly on smaller gauge long feeders. Arcing current is always less than fault current, and we also have examine the trip time at 85% of arcing current. There is a very high probability that the resulting value will 'NOT' be in the instantaneous range of the trip curve, and consequently IE values can be higher than 1.2 cals. I don't know how this practice got started but I don't agree with it.



I do not agree with you. But that is not important. The important side in this discussion is proof. I am vulnerable to data and research and resistant to opinions.

Just show me several cases (they do not have to be real situations in the field from one of your projects, just model them in your software) that circuits smaller than 100A (especially current limiting fuses) at 480V and at distances of less than 250 feet, will result in AFIE much larger than 1.2 cal/cm^2.

I have spent many hours researching this subject, modeling these circuits with many different available short-circuit currents and sizes and distances and concluded (and also with experience at the field) that it is extremely improbable a high AFIE in these small circuits. The length of the conductor must be larger than 250 feet to reduce the arc flash arcing current to a low value to enter the overload area of the breaker. The great majority of small branch circuits are less than 250 feet. Real case small ampere circuits with distances larger than 250 feet might experience unacceptable voltage drops and you would have to increase the wire sizes, bringing down again the impedance.

As you write, you do not like "assumptions" when calculating the AFIE. Me neither. But if you do not make these assumptions, you will never finish an AFHA.

The assumption of the utility must be made. Do you think that the value of short-circuit contribution and of voltage the utility gives you is exact and real?

Do you know that the impedance of the transformers might have a +- 7.5% tolerance. Which impedance you assume?

You have to assume the length of the conductors. Unless you know this length with no errors.

You have to assume that the OCPD will trip according to the published curves (assuming that they are maintained according to the manufacturer specifications).

You have to assume that the person will be exposed to the arc event not more than what you decided in the "maximum arcing time" field of your software.

Did you consider the reduction in contribution from your generators and did you recalculated the tripping time due to this lower current or you assumed a constant current from the synchronous generator.

Did you assumed that the upstream device (in case of mis- coordination) will trip before the device that should have tripped, giving you a lower AFIE if you did not assumed mis-coordination?

I can continue mentioning many more assumptions that you, me, and everybody decides to accept in order to perform an AFHA.

Many of these assumptions have a much greater impact on the result of the AFIE than the assumption that lower ampere rated devices will not produce AFIE much larger than 1.2 cal/cm^2.


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PostPosted: Sun Oct 27, 2013 4:40 am 
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Location: North Carolina
Transformers are specified with a +/-7.5% tolerance, then measured and the actual value (usually 1 or 2digits past the decimal) is stamped on the nameplate, at least all the ones I've ordered in the past 10 years bigger than a "stock" dry unit.


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